A search has disclosed various types of locking thread designs wherein a binding pressure is provided between the threads of male and female elements to produce a positive lock. Such prior art designs, for example, are shown in U.S. Pat. Nos. 1,657,244; 1,697,118; 1,798,604; 1,817,295 and 1,828,856 and in French Pat. No. 40,199 of 1932. These prior art locking thread designs are believed to operate satisfactorily in theory, but from a practical and commercial standpoint, are unacceptable due to the tolerance limitations under which modern thread forming equipment must operate. The locking thread design of the present invention on the other hand, may be readily formed with conveniently available equipment and in accordance with current tolerance limitations. More importantly, the thread forms of the present invention are not intended to lock by binding action, as is the case with the thread forms of the aforementioned prior art patents but instead, locking action is achieved by preventing relative motion between the mating parts and results in a locking action between the threaded elements that appears to be at least as good as, if not superior to the locking performance of the aforesaid prior art locking thread designs.